Carotenoids, such as astaxanthin, zeaxanthin, phoenicoxanthin, canthaxanthin, adonixanthin, and β-carotene, have been used as a naturally occurring pigment which is added to food, medicine, quasi-drugs, cosmetics and the like. In addition, because of their antioxidant properties, the carotenoids have also been widely used as an antioxidant substance. The carotenoids described above are widely distributed in plants, animals, and microorganisms, and several hundred types of natural carotenoids have already been identified (for example, see Non-Patent Document 1).
As microorganisms which synthesize functional carotenoids, a bacterium belonging to the genus Agrobacterium (which was later reclassified into the genus Paracoccus) was reported by Yokoyama et al (for example, see Non-Patent Document 2). The above bacterium is characterized by synthesizing astaxanthin, which is one of the functional carotenoids, at a high concentration. In addition, a biosynthetic pathway of astaxanthin by the above bacterium was investigated, and a detailed gene-level mechanism was reported (for example, see Non-Patent Document 3). According to the reported biosynthetic pathway, for the synthesis of carotenoids, mevalonic acid, which is a primary metabolite and which is used as a starting material, is processed through a metabolic synthetic pathway of isoprenoid biosynthesis to produce farnesyl pyrophosphate (FPP) having 15 carbon atoms, and the farnesyl pyrophosphate thus formed is then condensed with isopentenyl pyrophosphate (IPP) having 5 carbon atoms, so that geranylgeranyl pyrophosphate (GGPP) having 20 carbon atoms is synthesized. Next, two geranylgeranyl pyrophosphate molecules are condensed with each other, so that phytoene is synthesized. Phytoene is converted into lycopene by a series of desaturation reactions, and this lycopene is further processed by a ring-forming reaction, so that β-carotene is synthesized. Next, when β-carotene is oxidized by a ketonization reaction, canthaxanthin is produced, and when β-carotene is hydroxylated, zeaxanthin is produced. Furthermore, when oxidation reaction is further advanced by ketonization and hydroxylation, astaxanthin is synthesized. The carotenoid biosynthetic pathway described above is shown in FIG. 1.
In addition, it was also reported that some microorganism has no gene encoding the afore-mentioned enzyme which performs ketonization of β-carotene, and hence it selectively synthesizes zeaxanthin (for example, see Non-Patent Document 4). When the same scheme as described above can be applied to the case of canthaxanthin, that is, if a microorganism which has no gene encoding a β-carotene hydroxylase is present, canthaxanthin can be selectively synthesized; however, separation of a novel microorganism that selectively synthesizes canthaxanthin has not be reported as of today.
Canthaxanthin is an effective substance as a functional carotenoid to revive the colors of farm-raised fish, hen eggs, and the like; however, canthaxanthin is obtained from a carotenoid extract by purification and is then simply used. Since the carotenoid extract contains many carotenoids having chemical properties similar to those of canthaxanthin, extraction and purification thereof have not been easily performed (for example, see Patent Documents 1 and 2). Accordingly, a substance, such as a microorganism, which accumulates canthaxanthin at a high concentration, has been desired.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 6-237787
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2003-304875
[Non-Patent Document 1] Eric A. Johnson et al., Microbial carotenoids, Advances in Biochemical Engineering, Vol 53, pp. 119 to 178 (1995).
[Non-Patent Document 2] A Yokoyama et al., Production of astaxanthin and 4-ketozeaxanthin by marine bacterium, Agrobacterium aurantiacum, Biosci. Biotechnol. Biochem., 58: 1842 to 1844 (1994).
[Non-Patent Document 3] N. Misawa et al., Structure and functional analysis of a marine bacterial carotenoid biosynthesis gene cluster and astaxanthin biosynthetic pathway proposed at the gene level, J. Bacteriology, 177: 6575 to 6584 (1995).
[Non-Patent Document 4] L. Pasamontes et al., Isolation and characterization of carotenoid biosynthesis genes of Flavobacterium sp. strain R1534, GENE, 185: 35 to 41 (1997).